Proteasome inhibition fails to alter the kinetics of BCR-mediated antigen internalization. Splenic B cells were pretreated with the indicated proteasome inhibitor for 1 hour at 37°C, before analyzing the BCR-mediated internalization of bound ligand by flow cytometry, as described in “Materials and methods.” The results demonstrate that inhibition of proteasome activity fails to alter the kinetics of BCR-mediated antigen internalization, even though the treatment resulted in the accumulation of high-molecular-mass ubiquitinated proteins (Figure 2). Shown are representative results from 1 of 4 independent experiments (4 investigating the effect of MG-132 on BCR internalization, and 2 analyzing the effect of lactacystin treatment on BCR internalization).